An Interferometer Method of Studying the Vibrations of an Oscillating Quartz Plate

An expression involving the fringe brightness as a function of the amplitude of vibration is obtained for the case of a Michelson or a similar interferometer in which one of the returning mirrors executes S.H.M. in a direction perpendicular to its plane. A detailed discussion of the application of this expression to the measurement of the amplitude of vibration of a quartz plate is given, together with the observations upon four such plates. The superposition of either the first or second overtone upon the fundamental is also considered.The observed vibrations are predominantly flexural. The experimental results of Doerffler relating to the frequency dependence of the velocity of flexural waves are confirmed and found to apply to X-waves in a Y-cut. There is evidence of low frequency flexural waves propagated diagonally across a quartz plate.An interferometer method is suggested for distinguishing between flexural and longitudinal vibrations.